1. Field of the Invention
The present invention pertains in general to a nonvolatile memory device capable of having data electrically written therein, and more particularly, a nonvolatile memory device capable of selectively writing one of a plurality of values therein, and a method thereof.
2. Description of the Related Art
An Electrically Programmable Read-Only Memory ("EPROM") or a flash memory cell with a floating gate may be programmed by depositing electrons in the floating gate of the cell by electrons that tunnel from the cell substrate through a thin oxide layer disposed between the floating gate and the substrate. The tunneling electrons may be created by hot electron injection or Fowler Nordheim tunneling.
In a hot electron injection scheme, a high voltage is applied to the control gate of the cell and a low or zero voltage is applied to the drain region of the memory cell. Injection of electrons into the floating gate occurs when electrons in the channel region between the source and the drain regions of the cell attain an energy level higher than the barrier potential of the thin oxide layer. Some electrons will tunnel through the thin oxide layer and are injected into the floating gate, thereby depositing charges in the floating gate. For Fowler-Nordheim tunneling, a high voltage is applied to the control gate and a low voltage is applied to the drain region, and some electrons tunnel through the thin oxide layer to the floating gate.
However, not all electrons will attain an energy sufficient to tunnel through the thin oxide layer. The probability that an electron will tunnel through the thin oxide layer is proportional to the voltage difference between the control gate and the drain region. In addition, a charge proportional to the voltage difference between the control gate and drain will appear on the floating gate. The charge imposes an electric field on the channel region, known as the threshold voltage of the memory cell.
The threshold voltage determines whether a conventional memory cell is said to be storing any data or value. For example, a logic value of "0" is represented by setting a high threshold voltage and a logic value of "1" is represented by a low threshold voltage. Due to an increasing demand for memory storage capacity together with a decrease in available physical space, it is desirable to increase the memory storage capacity of individual memory cells. One attempt is described in U.S. Pat. No. 5,424,978 to Wada et al., which is hereby incorporated herein by reference.
Wada et al. describes a method and apparatus for storing more than two values within a single memory cell using hot electron injection. During programming, a high voltage pulse is sent to the drain of a memory cell. At the same time, a selected voltage from a stepped voltage signal is applied to the control gate. By applying the same voltage pulse to the drain at different times during the stepped voltage signal cycle, different levels of charges are stored in the floating gate. Therefore, each step of the stepped voltage signal produces a different threshold voltage in the memory cell, and, therefore, a plurality of "data" in the memory cell.
The method described in Wada et al. is limited in that the levels of the drain and control gate voltages are linked. For hot electron injection, the drain and control gate voltages must be set in a narrow requisite voltage range in relation to the other in order to create electrons having enough energy to tunnel through the thin oxide layer. The minimum requisite voltage is the threshold voltage of producing a charge on the floating gate, and the maximum requisite voltage is limited by the memory cell tolerance.
In general, the voltage applied to the control gate is fixed and the voltage applied to the drain is modulated to create different threshold voltage distributions. Because of cell tolerance, however, the voltage applied to the drain must be below the breakdown voltage to avoid the punch through effect. As a result, the allowable voltage difference between that applied to the drain and that applied to the floating gate is limited, thereby limiting the number of possible data that may be storage in the memory cell.